Quick answer
An AC that won't turn off is doing one of two things. If the outdoor unit keeps running with the thermostat set to Off, the contactor has welded shut or the thermostat circuit is shorted: switch it off at the breaker. If it runs non-stop but never reaches the setpoint, the controls are fine and the system is short on capacity.
When your AC won't turn off, set the thermostat to Off and wait five minutes before doing anything else. If everything stops, the system was only running because it could not reach the temperature you asked for, which is a capacity problem. If something keeps running, note which part: the indoor blower alone, or the outdoor unit as well. Each points to a different fault.
Outdoor unit running with the thermostat off: a welded contactor
If the outdoor unit runs with the thermostat off, the contactor is the first suspect. Its contacts carry the full compressor current, and after years of arcing they can fuse together and stay closed when the 24-volt signal is removed. The compressor and outdoor fan then run with no way for the thermostat to stop them, often with the indoor blower off.
That last detail is what makes it urgent. With no air moving across the indoor coil, it freezes solid and liquid refrigerant can return to the compressor. Turn the system off at the double-pole AC breaker in the panel, or pull the disconnect on the wall beside the unit, and leave it off. A contactor replacement runs $150–$350.
Only the indoor fan running: check ON versus AUTO
If air keeps coming from the registers but it is not cold and the outdoor unit is silent, the blower is running alone. Look at the fan setting on the thermostat: ON runs the blower around the clock, AUTO runs it only during a cooling call. Smart thermostats add circulation schedules that do the same thing for part of every hour. If the setting is AUTO and the blower still never stops, the fan relay on the furnace control board has stuck, a fault covered in why a furnace fan runs constantly.
A shorted thermostat cable or a stuck thermostat relay
The thermostat starts cooling by connecting R, the 24-volt supply, to Y, the cooling terminal. Anything else that joins those two wires does the same job without being asked. A staple driven through the cable, a picture-hanging nail, insulation chewed by mice in the basement ceiling or bare strands touching behind the thermostat will all hold the outdoor unit on.
There is a simple test. Pull the thermostat off its wall plate so it is disconnected. If cooling stops a minute or so later, the thermostat's own relay is stuck closed and the thermostat needs replacing. If the outdoor unit keeps running with the thermostat removed, the fault is in the cable or the contactor, and it is time to go to the breaker.
Running non-stop but never reaching the setpoint is a capacity problem
An air conditioner that runs all day and stops the moment you switch it off at the thermostat has no control fault. It is removing heat more slowly than the house gains it. A residential system around Burlington and Hamilton is sized to hold the house in the mid-20s on a design day of roughly 30 °C, so on a 35 °C afternoon with lake humidity it is expected to run continuously and may drift up a degree or two. Normal run times are set out in how long an AC should run per cycle.
The quick check is the temperature drop across the coil. Hold a thermometer at a return grille, then at a supply register near the furnace: the supply air should be roughly 8 to 11 °C cooler. A smaller drop on a mild day means the system has lost capacity.
- Low refrigerant charge from a leak: long run times, a hissing or bubbling sound, frost on the larger copper line. Leak repair and recharge runs $300–$800+ and has to be done by a technician holding an ODP card.
- Dirty condenser coil: the outdoor unit cannot reject heat through a mat of cottonwood fluff and grass clippings.
- Dirty filter or evaporator coil: low airflow, weak registers, sometimes ice.
- Undersized equipment: an addition, a finished attic or a new wall of west-facing glass adds load the original unit was never sized for. The second floor of a 1970s–90s two-storey is where it shows first.
- Extreme heat: a few hours of continuous running on the hottest July afternoons is the design working, not failing.
What to check first, and when to go straight to the breaker
- Thermostat fan setting: AUTO, not ON.
- Setpoint: 18 °C on a 33 °C day will never be reached; 24 or 25 °C will.
- Filter: replace it if you cannot see light through it.
- Outdoor coil: clear and clean on all four sides.
- Registers and return grilles: open and uncovered on every floor.
Skip the checklist, shut the breaker off and call now in any of these cases.
- The outdoor unit runs with the thermostat set to Off or removed from the wall.
- Ice is forming on the copper lines or the indoor coil.
- The breaker is the only thing that stops it.
- The unit ran all night and the house is still warming on a mild day.
Leave the breaker off until it is repaired; a compressor running against a frozen coil is the expensive way to find a $150–$350 part. A diagnostic visit is $90–$150 and is folded into the repair. ZK Mechanical handles AC repair across Burlington, Oakville, Milton, Hamilton and Mississauga with no weekend or holiday surcharge: call (647) 801-1252 or book online. Related faults are listed under cooling answers.